Americans are having a harder time falling asleep and staying asleep every year, a new study reveals.
Researchers found that as many as five million US adults struggle to sleep peacefully throughout the night.
Surprisingly, difficulty falling and staying asleep were most common in those who slept the recommended seven to nine hours most nights.
The team, from Iowa State University, hypothesizes - based on past research that shows teens have trouble falling asleep when they look at their phones before bed - that the behavior may be passing on to adults.
The National Sleep Foundation recommends that adults get between seven and nine hours of sleep every night.
However, a 2015 study from the Centers for Disease Control and Prevention found that about 50 percent of US adults sleep fewer than the recommended hours.
Insufficient sleep has been shown to raise the risk of obesity, diabetes, heart disease and even cancer.
For the new study, published in the journal Sleep Health, the team looked at data from about 165,000 people from the National Health Interview Survey.
Between 2013 and 2017, the number of adults who reported trouble falling asleep at least one day per week increased by 1.43 percent.
And the number of adults who said they had trouble staying asleep at least one day per week rose by 2.7 percent.
The percentages may not seem significant, but the researchers say this means as many as five million Americans are struggling to get a good night's sleep.
'Indeed, how long we sleep is important, but how well we sleep and how we feel about our sleep is important in its own right,' said Dr Zlatan Krizan, a professor of psychology at Iowa State University.
'Sleep health is a multidimensional phenomenon, so examining all the aspects of sleep is crucial for future research.'
The team couldn't definitively say what's causing sleep quality to decline, but they suggest technology may be to blame.
'We know from our previous research there is a correlation between smartphone use and insufficient sleep among teens,' said lead author Garrett Hisler, a former Iowa State graduate student and now a postdoctoral researcher at the University of Pittsburgh.
'If we're on our phone before bed or we're receiving alerts in the middle of the night that can make it harder to fall asleep and stay asleep throughout the night.'